Electronic display devices, such as monitors used with computers and screens built into telephones and portable information devices, are usually designed to have a viewing angle as wide as possible, so that they can be read from any viewing position. However, there are some situations where a display that is visible from only a narrow range of angles is useful. For example, one might wish to read a private document using a portable computer while on a crowded train.
There are a number of articles disclosing “hybrid aligned nematic” (HAN) and “chiral hybrid aligned nematic” (CHAN) liquid crystal (LC) cells. A hybrid aligned cell has vertical (homeotropic) alignment on one substrate)(pretilt=90° and planar alignment on the other substrate)(pretilt<10°.
Mash et al describe a CHAN LC cell for use as a pointer dial with no moving parts in U.S. Pat. No. 4,114,990 (1978). Lewis et al describe guiding of polarised light in a CHAN cell (Applied Physics Letters, Vol. 51, pp. 1197). Jewell et al describe director profile structures in CHAN cells filled with a dual frequency LC material (Phys Rev E, Vol. 73, 011706).
There are a number of articles disclosing the LC “pi-cell” or optically compensated birefringent (OCB) mode operation. A pi-cell is constructed from two substrates that have substantially identical pretilt alignment (both substrates usually induce planar alignment). The pi-cell is constructed with the rubbing directions parallel.
Bos et al describe fast switching in pi-cells (Mol. Cryst. Liq. Cryst., Vol. 113, pp. 329, patents U.S. Pat. No. 4,635,051 and GB 2 276 730.)
U.S. Pat. No. 6,552,850 (E. Dudasik; Citicorp Inc. 2003) describes a method for the display of private information on a cash dispensing machine. Light emitted by the machine's display has a fixed polarisation state and the machine and its user are surrounded by a large screen of sheet polariser which absorbs light of that polarisation state but transmits the orthogonal state. Passers by can see the user and the machine but cannot see information displayed on the screen.
A versatile method for controlling the direction of light is a ‘louvre’ film. The film consists of alternating transparent and opaque layers in an arrangement similar to a Venetian blind. Like a Venetian blind, it allows light to pass through it when the light is travelling in a direction parallel or nearly parallel to the layers, but absorbs light travelling at large angles to the plane of the layers. These layers may be perpendicular to the surface of the film or at some other angle. Methods for the production of such films are described in a U.S. RE27617 (F. O. Olsen; 3M 1973), U.S. Pat. No. 4,766,023 (S.-L. Lu, 3M 1988), and U.S. Pat. No. 4,764,410 (R. F. Grzywinski; 3M 1988).
Other methods exist for making films with similar properties to the louvred film. These are described, for example, in U.S. Pat. No. 5,147,716 (P. A. Bellus; 3M 1992), and U.S. Pat. No. 5,528,319 (R. R. Austin; Photran Corp. 1996).
Louvre films may be placed either in front of a display panel or between a transmissive display and its backlight to restrict the range of angles from which the display can be viewed. In other words, they make a display “private”.
US 2002/0158967 (J. M. Janick; IBM, published 2002) shows how a light control film can be mounted on a display so that the light control film can be moved over the front of the display to give a private mode, or mechanically retracted into a holder behind or beside the display to give a public mode. This method has the disadvantages that it contains moving parts which may fail or be damaged and that it adds bulk to the display.
A method for switching from public to private mode with no moving parts is to mount a light control film behind the display panel and to place a diffuser which can be electronically switched on and off between the light control film and the panel. When the diffuser is inactive, the light control film restricts the range of viewing angles and the display is in private mode. When the diffuser is switched on, it causes light travelling at a wide range of angles to pass through the panel and the display is in public mode. It is also possible to mount the light control film in front of the panel and place the switchable diffuser in front of the light control film to achieve the same effect.
Switchable privacy devices of these types are described in U.S. Pat. No. 5,831,698 (S. W. Depp; IBM 1998), U.S. Pat. No. 6,211,930 (W. Sautter; NCR Corp. 2001) and U.S. Pat. No. 5,877,829 (M. Okamoto; Sharp K. K. 2001). They share the disadvantage that the light control film always absorbs a significant fraction of the light incident upon it, whether the display is in public or private mode. The display is therefore inefficient in its use of light. Since the diffuser spreads light through a wide range of angles in the public mode, these displays are also dimmer in public than in private mode, unless the backlight is made brighter to compensate.
Another disadvantage relates to the power consumption of these devices. In the public mode of operation, the diffuser is switched to a diffusing state. This often means that voltage is applied to a switchable polymer-dispersed liquid crystal diffuser. More power is therefore consumed in the public mode than in the private mode. This is a disadvantage for displays which are used for most of the time in the public mode.
Another known method for making a switchable public/private display is disclosed in U.S. Pat. No. 5,825,436 (K. R. Knight; NCR Corp. 1998). The light control device is similar in structure to the louvre film described earlier. However, each opaque element in the louvre film is replaced by a liquid crystal cell which can be electronically switched from an opaque state to a transparent state. The light control device is placed in front of or behind a display panel. When the cells are opaque, the display is in its private mode; when the cells are transparent, the display is in its public mode.
The first disadvantage of this method is in the difficulty and expense of manufacturing liquid crystal cells with an appropriate shape. A second disadvantage is that, in the private mode, a ray of light may enter at an angle such that it passes first through the transparent material and then through part of a liquid crystal cell. Such a ray will not be completely absorbed by the liquid crystal cell and this may reduce the privacy of the device.
Another method for making a switchable public/private display device is disclosed in JP3607272 (Toshiba 2005). This device uses an additional liquid crystal panel, which has patterned liquid crystal alignment. Different aligned segments of the panel modify the viewing characteristics of different areas of the display in different ways, with the result that the whole display panel is fully readable only from a central position.
GB2405544 describes switchable privacy devices based on louvres, which operate only for one polarisation of light. The louvres are switched on and off either by rotating dyed liquid crystal molecules in the louvre itself or by rotating the plane of polarisation of the incident light using a separate element.
In GB2413394, a switchable privacy device is constructed by adding one or more extra liquid crystal layers and polarisers to a display panel. The intrinsic viewing angle dependence of these extra elements can be changed by switching the liquid crystal electrically in the well-known way.
In GB2410116, a display is switched from public to private mode by using two different backlights which generate light with different angular ranges.
In GB2421346, a polarisation modifying layer (PML) is placed behind the exit polariser of a liquid crystal display panel. Some parts of the PML are simply transparent. Other parts change the polarisation of light passing through them so that pixels viewed through these parts are inverted in colour (bright pixels becoming dark and dark pixels becoming bright). Data sent to pixels directly behind these parts is inverted so that, when the display is viewed from a central position, the image appears normally. However, when the display is viewed from a different angle, different pixels are viewed through the retarder elements and the image is corrupted. Off-axis viewers see a confusing image which is a random dot pattern. The PML may be made from liquid crystal and may be switched off to give a public mode.
GB2418518 adds a guest host (dyed) LC layer with a patterned electrode to a standard thin film transistor (TFT) LC display. The dyed LC layer can be switched between an absorbing state (private) and non-absorbing state (public). The dye molecule absorption is dependent upon the incident angle and polarisation of light. For a given polarisation and orientation, the absorption of the dye increases with larger viewing angles resulting in low brightness at high angles (narrow mode).
GB2426352 discloses the combination of a privacy function and a three dimensional (3D) function provided by a single additional switch cell. The display has three operating states, a wide mode, a private mode and a 3D mode. Both patterned and unpatterned LC alignment embodiments are described.
The concept of using a hologram to provide a privacy function is disclosed in GB2404991 but such a display suffers from two disadvantages. First, due to unwanted diffraction of light from the display by the hologram, the colour of the image seen by viewers is incorrect. Second, for applications using a touch screen mounted on the front of the display, the user's hand can block the illumination of the hologram and so reduce the effectiveness of the privacy. GB2428128 discloses solutions to these problems.
GB2427033 discloses the use of an extra liquid crystal layer located between the existing polarisers of a liquid crystal display (LCD) panel. In this location, the extra switch cell can modify the greyscale curves for off axis light. This provides a higher level of privacy for images than the techniques disclosed in GB2413394.
GB patent application no. 0613462.1 discloses the use a switchable privacy device constructed by adding an extra cholesteric layer and circular polarisers to a display panel. The cholesteric layer can be switched between a public (wide view) mode and a private (narrow view) mode that can provide 360° azimuthal privacy.
Adachi et al (SID06, pp. 228) and Okumura (US20050190329) disclose the use of a HAN cell to provide a switchable privacy function. The HAN cells used by Adachi and Okumura are used in conjunction with an underlying image panel. The public (wide view) modes described by Adachi et al (SID06, pp. 228) and Okumura (US20050190329) are untwisted.
Kubono et al investigate switching speeds of CHAN cells that comprise a low pretilt substrate and a vertically (homeotropically) aligned substrate rubbed at 60°, 90° and 120° with respect to the low pretilt (planar) substrate (Journal of Applied Physics, Vol. 90, pp. 5859). The liquid crystal material used by Kubono et al has been chirally doped.
JP09230377 and U.S. Pat. No. 5,844,640 describe a method of changing the viewing angle properties of a single layer LCD panel. This is achieved for a Vertically Aligned Nematic (VAN) LC mode. Electric fields in the plane of the display panel are used to control how the LC material tilts in a pixel area. The number and orientation of different tilt domains within a pixel can be controlled by the in-plane fields. A pixel with several tilt domains will have a wide viewing angle; a pixel with one tilt domain will have a narrower viewing angle. The use of this method to vary the viewing angle of a display is described. However the viewing angle of a single tilt domain of the VAN mode described is not sufficiently narrow to provide good privacy.
U.S. Pat. No. 5,686,979 (3M) describes the use of reflective polariser films or “dual brightness enhancement films” (DBEF) and an additional switch LCD for use in conjunction with an existing image LCD. The optical system can be switched between a transmissive LCD mode and a reflective LCD mode. In the reflective LCD mode, the ambient light is used instead of a backlight to display an image on the existing image panel.
Switchable mirror display patents EP 0 933 663 B1 (Citizen) and JP34197661 (Hitachi) describe the use of reflective polariser films (DBEF) and an additional LCD for use in conjunction with an existing image LCD. This optical system can be electrically switched between two modes of operation: 1) a public (wide view) mode whereby the additional LCD and DBEF film do not alter the image from the existing image LCD; 2) a mirror mode whereby ambient light is reflected from the DBEF to produce a mirror mode.